2007 OPEN FORUM Abstracts
DEMAND-FLOW IMPROVES MAINTANANCE OF SPONTANEOUS BREATHING DURING HFOV IN PIGS
K. Roubik1, M. van Heerde2, V. Kopelent1, D. G. Markhorst2
Objectives: Both experimental models and clinical studies emphasize the positive effect of spontaneous breathing during mechanical ventilation. The aim of the study was to evaluate effect of spontaneous breathing during HFOV using demand-flow (DF) instead of continuous flow (CF) of fresh gas through the HFOV ventilator.
Methods: Prospective comparative interventional study in an animal model. Equipment: A custom designed demand-flow system (DFS) was assembled using an electrically-driven computer-controlled flow valve, an airway pressure sensor with a measuring circuit and a multi-channel AD/DA converter. DFS is controlled by real-time PC software separating the HFOV ventilator pressure swings from the patient-generated pressure changes. The patient-generated signal is used for control of DFS with the aim to minimize these pressure changes. Subjects: Eight intubated, sedated but spontaneously breathing pigs weighing 50 kg were used. Lung injury was induced by repeated lung lavage with normal saline. Thereafter, HFOV (3100B, SensorMedics) was initiated. Measurements were performed using a three-step protocol comprising different HFOV modes lasting 30 min each: (1) HFOV and spontaneous breathing maintained with CF (e.g. using an unmodified ventilator); (2) HFOV and spontaneous breathing maintained with DFS; (3) HFOV with suppressed spontaneous breathing. Pressures were sampled in the ventilator circuit, trachea and esophagus in order to calculate different components of work of breathing (WOB) and respiratory mechanics. Arterial and mixed venous blood was sampled to evaluate gas exchange.
Results: Tidal volume (Vt) and minute ventilation (MV) of spontaneous breathing were higher during DF compared to CF: Vt = 296 ± 31 vs. 196 ± 13 mL (p<0.001), MV = 5.8 ± 1.0 vs. 4.2 ± 0.7 L/min (p<0.001). Using CF, imposed WOB caused by the ventilator was excessively high (0.42 ± 0.05 J/liter), WOB with DF was reduced by 83% (to 0.071 ± 0.01 J/liter, p<0.001). Oxygenation and ventilation, measured by arterial pO2 and pCO2, were best preserved with DFS, followed by CF. If spontaneous breathing was suppressed, oxygenation and ventilation deteriorated.
Conclusions: DFS improves maintenance of spontaneous breathing during HFOV in pigs. Work of breathing is significantly reduced. DFS did not cause excessive large tidal volumes. Gas exchange was enhanced if spontaneous breathing was preserved.
Acknowledgment: Supported by grant MSM 6840770012.